Small flexible or semi-flexible endoscopes, like a uretheroscopes, uretero-renoscopes or other small flexible and semi-flexible instruments, have an outer diameter that ranges from 1 mm to 3 mm. Within the small area defined by the cross-section of such scopes there are illumination bundles for illuminating an object to be inspected, an imaging system for transmitting an image of the illuminated object to the physician, channels through which instrumentation and fluid irrigation can pass and an outer tube containing all these components. As a result, space within the outer tube is limited. In many instances, the diameter available within the outer tube for the imaging system is less or much less than a millimeter (mm).
Image bundles, which together with an objective form part of the imaging system, receive an image from the objective and transmit the image to an eyepiece or video chip for display to a physician. The outer diameter of the image bundles range from 0.16 mm to 0.85 mm. Based upon diffraction limits, these image bundles have pixels in the range of 1,600 to 30,000 with pixel sizes of not less than 5 μm to 61 μm. Compared to large instruments, these image bundles exhibit a low resolution, and therefore, the optical quality required of the objective for creating the distal image on the image bundle are also low.
Even though the optical quality required of the objective in systems utilizing image bundles having diameters ranging from 0.16 mm to 0.85 mm is low, it remains difficult to manufacture objectives for small scopes in a traditional manner. Lenses are traditionally made from glass lens blanks glued on a metal pin. The first optical surface of the lens is ground and polished. The lens is then removed and glued on a second pin where the second surface is ground and polished in the same way. Using this traditional method, objectives having a lens of 1 mm or more can be made.
To make lenses having a diameter of less than 1 mm, for example, as required in very small endoscopes, manufacturers start with glass balls in the sub-millimeter range. Balls of mineral glass or optical glass can be manufactured in large quantities in the sub millimeter range. Precision of the diameter of these glass balls are within optical tolerances, and the polished surfaces have optical quality. A large amount of these glass balls can be glued together on a tool and plano surfaces can be ground on one side of the balls. This remaining ball having a plano surface can be glued on a small pin and a cylinder surface can be ground on the periphery. This is a proven method for creating a simple plan convex lens in the sub millimeter range.
FIG. 1 depicts a conventional objective using two plano convex lenses 10, 12 made out of glass balls. The first lens 10 is an objective lens where the plano surface faces an object side 14 and is fixed in the head of a endoscope. The second lens 12 is a field lens facing an image bundle and is cemented on the image bundle (not shown). The image bundle with field lens 12 can be moved relative to objective lens 10 to adjust the focus of the objective system comprised by objective lens 10 and field lens 12. Light entering the objective is not limited, but the acceptance angle of the fibers in the image bundle has a limited numerical aperture which limits the f-number of the objective system.
For endoscopes used with instruments that are guided in an instrument channel of the endoscopes, the optical systems of the endoscopes are laterally displaced to the instrument channel. To have the instruments visible in the instrument channel as early as possible, optical systems often include a deflection component at the tip of the optical system. For example, referring to FIG. 2, some optical systems have a wedged surface 16 on a distal side on a front lens 18. In this case, the deflection angle depends highly on the optical parameters of the medium in which the endoscope is used. Often endoscopes are used in a liquid medium contained within a body cavity. However, the objective assembly is not immersed in the liquid so the deflection during viewing does not accurately represent the real application.
Referring to FIG. 3, there is depicted an objective with a wedge 20 facing the inside of the objective system. In this case, the deflection changes much less because the main deflection happens on the inside of the objective assembly where the distal medium has no influence. However, if the outside surface 21 of the lens is perpendicular, the optical axis is still slightly deflected when the distal medium changes. A corrected deflection in depicted in FIG. 4 where a surface 22 facing the distal medium is angled so that the surface is perpendicular to the optical axis of a wedge surface 24. However, the technical difficulties with his arrangement are high compared to the advantages. Accordingly, the preferred angulation for smaller optical systems in endoscopes is as represented in FIG. 3.
The limitation of the brightness through the amount of light accepted by the imaging fibers has disadvantages. Any light entering the objective system but not accepted by the imaging fibers results in glare and reflections. Aperture stops used to avoid the entrance of light in the optical system which is not used in the imaging system. A prior art method of applying an aperture is to deposit a non-transparent coating on a glass plate with a circular opening in the middle. Such aperture stops can be produced in very high accuracy and quantity on one glass plate. The individual apertures are then separated and ground down to the right diameter. A preferred application is an aperture stop on a glass plate cemented on the plano side of a plano convex objective lens where the aperture stop faces the plano surface of the lens.
A current deflective objective assembly including a wedge 26 and an objective 28 with an aperture stop 30 is depicted in FIG. 5. The integrated aperture stop 30 allows for reduction of the effective diameter of objective lens 28 compared to the diameter of the image bundle and field lens 32 to which the objective is coupled. As a result, wedge 26 and objective lens 28 with aperture stop 30 can have a smaller diameter than field lens 32. As displayed in FIG. 6, since wedge 26 and objective 28 have a smaller diameter, they can be contained within a small sleeve 34 having an outer diameter that is that is equal to the outer diameter of the image bundle and field leans 32. Sleeve 34, containing wedge 26 and objective 28, can then be glued as a unit within the distal objective head of an endoscope. A shortcoming of this combination is that its very difficult to assemble in the objective head.
The fact that these subassemblies of wedge and objective lens are in the range of a few tenth of a millimeters poses the risk that the small subassemblies, when glued in the distal tips of endoscopes, will not always be perfectly sealed. To improve the quality of such seals, it is known to glue a front window made from mineral glass or resistant hard optical glass to the object side of the subassemblies. Such subassemblies are shown in FIGS. 7 and 8. FIG. 7 depicts a configuration where a front window 31 is glued to a sleeve 34 containing a wedge 36 and an objective lens 38 with an aperture stop 40. Sleeve 34 is extended in the direction of the image side so that it can act as a guide for a field lens 42. In this arrangement, wedge 36 and objective 38 each have an outer diameter that is the same as the outer diameter of field lens 42. FIG. 8 illustrates a similar configuration but with a shorter sleeve 44 and a larger diameter field lens 46. In this arrangement, sleeve 44 can be glued in the distal tip of the endoscope, and field 46 lens can be moved freely with the image bundle relative to the objective for focusing an image onto field lens 48.
Each of the objective assembly designs described above include shortcomings that make the assembly of them difficult and their rate of failure relatively high. This is especially true for objective assemblies having a large field of view where the ray heights increase with the distance to the aperture stop. Thus, even with axial thicknesses of a few tenths or less, the accumulation of optical components within the assembly can result in relative large ray heights at the front surface and the field lens. Also through asymmetric deflection, ray heights on one side of the front window do not increase proportionally resulting in glare, reflections or cut-off of the image.